This invention relates to the machining of components comprising a substantially electrically non-conductive layer applied to an electrically conductive substrate. In particular the invention concerns a method and apparatus for machining through an undoped ceramic material coating applied to a metal substrate.
Electro discharge machining (EDM) is well known as a method of drilling small holes through metal components such as gas turbine blades and guide vanes. In an EDM process pulses of positive electrical potential are applied to an electrode held close to the surface of a component where a hole is to be drilled while the component is negatively biased. Dielectric fluid is supplied to the gap between the component and the electrode and a succession of voltage pulses are applied to the electrode to produce the machining sparks.
EDM enables a multiplicity of holes to be drilled simultaneously using a multi-wire head. The process is relatively quick, cheap, accurate and produces an acceptable finish in the super alloy metals normally used for gas turbine components. However, EDM cannot be used to form holes through electrically non-conductive ceramic because of its insulating properties.